The invention relates to maskless lithography, and relates in particular to maskless lithography using arrays of diffractive focusing elements.
Diffractive focusing elements differ from refractive focusing elements such as ordinary glass lenses in that the directing of incident radiation to a focus is accomplished by means of constructive interference of waves emanating from multiple regions of the element, with those regions organized in shape and location so as to accomplish said constructive interference. The simplest example of a diffractive focusing element is a pinhole, and diffractive focusing systems using pinholes may be used, for example, in pinhole cameras, in the imaging of gamma rays in medicine, or in the focusing of high energy particles.
U.S. Pat. No. 5,900,637 discloses a system for maskless lithography based on an array of diffractive focusing elements known as Fresnel zone plates. In particular, U.S. Pat. No. 5,900,637 discloses the use of various diffractive focusing elements for applications in which the source produces an electromagnetic field (e.g., x-rays) that would be absorbed by traditional refractive focusing elements. Such diffractive focusing elements disclosed in U.S. Pat. No. 5,900,637 include amplitude Frensel zone plates (which consist of alternating opaque and transparent regions), Fresnel phase plates (which consist of alternating transparent regions that provide a relative phase shift of 180 degrees), and mixed Fresnel zone plates (which consist of alternating regions that provide both alternating amplitudes and relative phase shift).
Although such diffractive focusing elements may be used in lithography systems employing radiation other than x-rays, amplitude Fresnel zone plates typically provide an efficiency of focusing incident radiation into the first-order focus of approximately 10%, while Fresnel phase plates typically provide an efficiency of focusing into the first-order focus of approximately 40%. Higher efficiencies are highly desirable for most lithographic applications. Moreover, some applications, such as the focusing of pure x-ray wavelengths, are not possible using only Fresnel phase plates, requiring the use of mixed Fresnel zone plates.
There remains a need therefore, for a diffractive focusing system that may be used more efficiently and economically in a maskless lithography system.